Computer systems, including the increasingly popular personal computers (PC's), frequently employ various types of peripheral equipment such as printers, plotters, machine tools, and the like, for responding to data in the computer to perform various operations in response to programs processed by the computer. To provide a communications link between the computer and these various pieces of peripheral equipment, typically, an output "port" of the computer is connected by means of a cable to the particular utilization device which is to be operated. For example, a serial output port known as the RS-232 port frequently is interconnected by means of a cable to a printer for effecting the transfer of data and control signals from the computer to the printer. Data signals travel over one set of wires in this cable and control signals for controlling the operation for the printer in accordance with the instructions supplied by the computer are supplied over a separate lead or set of wires in the cable. The RS-232 serial output port, and similar output ports (both serial and parallel for various types of computers, are capable of interfacing with only a single serial output device when they are interconnected in this manner.
In an effort to provide a capability of the computer to communicate with different serial output devices, one solution is to simply provide additional dedicated serial output ports. In such a situation, a first output port is connected to a printer, a second output port is connected to a plotter, a third output port is connected to a machine tool, and so forth. This considerably increases costs, however.
Another approach, which presently is available, is the provision of a manual multi-device serial switch block which interfaces between the various different serial devices and a single serial output port of a computer. In such a system, operation of a switch interconnects the serial output port of the computer with a selected one of the utilization devices, each of which are connected by means of a separate cable to the switch block. While this solution to the interface problem is relatively straightforward, it is not particularly "user-friendly", inasmuch as the user must manually determine the correct interconnection between the computer and the desired serial output device each time data is to be passed from the computer to such a device. If the wrong switch is selected or the switch setting is improper, the desired communication does not take place. The user generally is provided with a ready indication of such an error by the computer. The user then must make the proper connection and operate additional keys on the computer to reinitiate the desired communication interface. In addition, if a relatively large number of potential serial output devices are to be controlled by a single output port of a computer, a large number of switches or a relatively complex switch arrangement is required. This further increases the chances for error by the operator in selecting the wrong switch.
In place of employing a manually operated multiple switch to select a desired one of a number of different serial output devices for interconnection with the serial output port of a computer, systems have been developed which connect a computer output data line or bus in common to a number of potential utilization devices and which then employ a separate address bus interconnecting the computer with the different output devices. Unique address signals are sent over the address bus to activate the selected one of the different output devices for receipt of the information on the data bus.
A patent which discloses this type of system is the patent to Price U.S. Pat. No. 3,445,813. The system of Price employs one set of conductors for data, a separate set of connectors for address codes, and a third set of connectors for supplying power from a central computer to a number of different outlying stations. Each of these conductors are bundled together in a cable which is interconnected with all of the different outlying stations. Power is supplied over the power cables to activate at least the address signal decoding portions of the different stations and when an address code uniquely identifying one of those stations is received by such a station, that station is turned on to respond to the information present on the data or information conductors.
Three other patents employing systems which are similar to Price are the patents to Kobayashi U.S. Pat. No. 3,986,169; Crandall U.S. Pat. No. 4,048,620; and Sem-Sandberg U.S. Pat. No. 4,514,841. The systems of all of these patents are similar to one another. None of them is specifically directed to the unique configuration of the output ports of personal computers, such as the RS-232 or 449 ports. In addition, these systems require a relatively large number of connectors for interconnecting the various outlying stations with the central computer.
It is desirable to provide a simple automatically operated system for selecting one of a number of output utilization devices for control and the supplying of data from a single output port of a computer, such as a personal computer (PC).